Fixed Strict Error by PHP 5.4 in SoapSource

PHP 5.4 was throwing the following Strict Error:

Declaration of SoapSource::listSources() should be compatible with
DataSource::listSources($data = NULL) in [..app/Plugin/Datasources
/Model/Datasource/SoapSource.php, line 27]
Merge pull request #45 from netors/2.0

Fixed Strict Error by PHP 5.4 in SoapSource